Increased storage capacity in magnetic recording has traditionally been addressed through improvements in the ability to store information on a particular storage disc with an increased areal density, e.g., decreasing the size of the inductive write element and read back sensor in a magnetic recording system. Until recently, these prior art approaches have been adequate for increasing the storage capacity of magnetic recording discs.
Typically in magnetic recording, the magnetic vectors of ferromagnetic domains in a storage medium are arranged in a coherent manner to store data. For example, if the vector direction between adjacent domains is reversed, a binary “1” can be stored.
The areal density in magnetic recording technologies has now reached 60 to 70 Gbit/in2 in certain magnetic storage medium, and is increasing at a rate of between 60% and 100% per year. Further, data rates are increasing at a rate of approximately 30% to 40% per year. However, one limitation of magnetic storage technologies is due to the ferromagnetic domains themselves. As the size of the ferromagnetic domains are reduced in the storage medium in order to achieve higher packing densities, the anisotropy energy of the magnetic domain decreases. Below what is known as the “superparamagnetic” limit, the thermal energy can overcome the magnetic anisotropy such that it is not possible to record data.
Ferroelectric materials also have domains. However, with ferroelectric materials, the domains are formed by charged regions rather than magnetic vectors. Ferroelectric domains can be formed much smaller than magnetic domains and are capable of yielding much higher storage densities than magnetic storage mediums.
Various techniques can be used for reading back data stored on a ferroelectric storage medium. One technique which can be used to readback data uses the piezoelectric properties of the storage medium. However, this technique cannot operate at the high frequency necessary for high data rates, for example above 1 MHz. Another technique uses a scanning nonlinear dielectric microscope (SNDM) in which a lock-in amplifier is used to measure the nonlinear dielectric properties of the storage media. However, this technique also suffers from limited data rates because the lock-in sampling rate must be approximately ten times the data rate.